The advantages of the pressure reactor for the production of DAP were discovered while working with a pipe-cross reactor (PCR) in a commercial granulation plant. This commercial plant uses the conventional type TVA process for the production of DAP. The reactor which is used is constructed of 316L stainless steel. The original reactor was 10 inches in diameter and 15 feet long. Currently a 6-inch diameter reaction tube with a 4-inch circular pipe discharge is being used.
The plant has a throughput capacity of about 500 tons per hour. In a production test, a rate of 98 tons per hour was maintained for several days. Results from this test are shown in Table I below.
TABLE I ______________________________________ Production of DAP Using PCR and Preneutralizer B6981 Test No. Ton/hr ______________________________________ Production rate 98 Formulation, product (lb/ton) To scrubbers Weak phosphoric acid (21.1% P.sub.2 O.sub.5) 925 Strong phosphoric acid (44.2% P.sub.2 O.sub.5) 1566 Calculated acid strength, % P.sub.2 O.sub.5 37.5 To preneutralizer Scrubber liquor (N:P mole ratio 0.80) 1940 Ammonia (gaseous) 122 To PCR Scrubber liquor (N:P mole ratio 0.80) 574 Ammonia (gaseous) 38.8 Calculated % total production for PCR 23 Production rate PCR 22.5 To granulator 122 Ammonia (gaseous) Operating results Preneutralizer N:P mole ratio 1.60.sup.a Temperature slurry, .degree.F. 257 Specific gravity slurry 1.57 PCR N:P mole ratio 1.54.sup.b Temperature melt, .degree.F. 320 Pressure, (psig) 51 Specific gravity 1.54 Granulator N:P mole ratio 1.92.sup.c Material discharge temperature, .degree.F. 212 Calculated results Recycle ratio (from screen analysis) 2.0 tons recycle/ton product Heat flux PCR (Btu/hr. in.sup.2) 135,000 Ammoniating volume (lbs NH.sub.3 /hr/in.sup.3) 0.4 ______________________________________ .sup.a Obtained by filtration during operation (average). .sup.b Based on chemical analysis of sample. .sup.c Chemical analysis of composite sample.
As may be seen in Table I, supra, weak phosphoric acid containing 27.1 percent P.sub.2 O.sub.5 was fed to the scrubber along with some medium strength acid containing 44.2 percent P.sub.2 O.sub.5. Average strength of the acid fed to the scrubber was about 38 percent P.sub.2 O.sub.5. This scrubber slurry was fed both to the preneutralizer and to the PCR. Calculations show that about 23 percent of the total production was made by the PCR and the remainder was made through use of the preneutralizer. The proportions of phosphoric acid fed to the scrubber and to the preneutralizer resulted in an N:P mole ratio of 0.80:1 in the scrubber slurry. Ammonia was added to the preneutralizer and to the pressure reactor. The average recorded feed rates showed that the N:P mole ratio of the slurry from the preneutralizer and PCR should be about 1.54:1. Chemical analysis of grab samples of this slurry shows the mole ratios to be somewhat higher than the desired ratios. Temperature of the slurry from the preneutralizer was 257.degree. F. and from the PCR 320.degree. F. at a pressure of 51 psig. Because the reactor is operated under pressure, it is possible to increase heat content of the melt discharged from the reactor as compared to the slurry sprayed from the preneutralizer.
The plant operated well under these conditions and a nominal 18-46-0 DAP was produced which had good particle size distribution and excellent hardness. The recycle:product weight ratio in this test appeared to be substantially less than the 5:1 recycle ratio normally required in DAP plants. Moisture content of the product from the granulator was only 2.9 percent, which is significantly less than the 4 percent moisture normally obtained for this product when the TVA conventional DAP process is used. Unfortunately, there was low airflow through the granulator during this test. However, in spite of this adverse condition, this company has experienced an increased production rate from about 1,800 tons per day before the pressure reactor was installed to about 2,400 tons per day after its installation. It is believed that, although the pressure reactor operation does not account for all of the increased production rate, it is the major factor. Also, some savings in the amount of fossil fuel required to dry the product were observed. Estimates of these savings of fuel requirement run as high as about one dollar per ton even though the pipe was used in conjunction with the preneutralizer tank. Note: These savings are, of course, much greater when the pipe is used by itself in the practice of the instant invention, to wit, no fuel is required for drying.